How thick is the plate you used for the door ?
It's only 10ga, the same as the rest of the FB. My neighbor got it for me and that was what he had. The price was right. I figured that it would just be more susceptible to cold weather and maybe rust out sooner. I didn't figure on it moving this much when it was hot. The rest of the FB moves, but it's not as obvious. I framed the opening out with 3/4in angle, and that seems to stay put.
The place I bought all the rest of my steel from, only deals in full sheets. So I would be looking at a 4x8 sheet of 1/4in plate, and I've got no way to deal with that. I got a reasonable sized piece of 3/16 for the RF plate, but that was a friend of a friend kind of thing.
Scrap yards, welding shops, steel erectors and fabricators all should have some pieces of plate" drops " lying around or can tell you where to find some. I posted a couple ideas on your PM, If that doesn't work a thicker piece of plate for the door would be my next step.
Ok so I've not been able to get as much done as I wanted to. It's been pretty cold here and my garage is unheated. So my motivation isn't what it should be. Combine that with all the holiday stuff and the kids and you get some slacking going on.
That said, I got the upper air vent cut in and functional, just not completely finished yet, and I built a proof of concept on the firebox door latch. Then I threw a couple of picnics on it and did another smoke. The Firebox door latch worked well enough to keep the warping to a minimum. A couple of times when I opened it up to feed it, the door sort of popped, but was easy enough to get back closed tight. So I'm going to go ahead and finish up the latch as I was planning.
The upper air vent made a huge difference. I can hardly believe it. At one point I had the main vent about 80% closed, the upper vent about 2/3rds closed, and the CC was pretty steady at 275. Just got to remember to close it when I open the FB door, I had flames shooting out of it at one point. It still burns an unbelievable amount of fuel, but thats just life I suppose.